The present invention relates to a method for data transfer in a digital mobile communications network, in which method it is handled user data in certain layers according to certain protocols, in a certain layer of said layers it is transferred user data over a physical radio channel between a mobile station and a fixed mobile communications network in radio blocks, for the transfer of said layer it is formed in the radio block a payload of a certain size comprising check bits connected with the performing of the transfer and transfer bits available for the transfer of user data, each radio block is channel coded using a certain coding method and the size of said payload is dependent on the coding method. The invention also relates to a transmitter/receiver device operating according to the method and a mobile communications system. The invention relates in particular to data transfer in the GSM-system in the GPRS-packet switched service.
Out of the present mobile communications systems a majority is offering data- and voice services based upon circuit switched technique. In the circuit switched technique a transfer connection is maintained during the whole connection even if no information would be transferred time to time. This unnecessarily consumes the transfer resources, shared by also many other users, in which case the maintaining of a circuit switched transfer connection to one user consumes unnecessarily the transfer resources of other users. Because of the bursts in the GSM-transmission, data services are not optimal in the circuit switched technique. However, the packet switched information transfer for the increasing of the efficiency of the utilization of a channel is known.
As well as the fixed network also a future mobile communications network must be able to transfer both circuit switched and packet data transfer, e.g. ISDN-transfer (Integrated Services Digital Network) and ATM-transfer (Asynchronous Transfer Mode). For information transfer using packet switching it is known in mobile communications systems a protocol based upon packet reservation multiple access called PRMA (PRMA, Packet Reservation Multiple Access). It is also spoken of as packet radio. PRMA is a technique for multiplexing digital speech or data in a time division carrier wave, i.e. PRMA uses in a radio channel a time division multiple access (TDMA, Time Division Multiple Access), in which case transmission and reception take place at certain moments using time division. The PRMA-protocol has been developed to utilize the discontinuity of speech transfer in order to support more users than the number of speech channels in a time division carrier wave. In such a case a channel is allocated to a mobile station, for example a speech channel when speech is being produced and when the speaking ends the channel is released, in which case the mobile station does not unnecessarily reserve capacity, but the channel is channel is free for other purposes, for example the transmissions of other mobile stations in the cell. The PRMA-protocol is used in cellular mobile communications systems in the communication between a mobile station and a base station. The GSM GPRS (General Packet Radio Service)-system is an example of a system based upon a PRMA-type protocol.
GPRS is a new GSM-service, by using which the packet radio operation can be made available to GSM-users. GPRS reserves radio resources only when there is something to transmit, in which case the same resources are shared between all mobile stations as needed. The normal circuit switched network of the GSM-system has been designed for circuit switched speech transmissions. The main goal of the GPRS-service is to realize the connection from a mobile station to a public data network using prior known protocols, such as TCP/IP and X.25. However, there is a connection between the packet switched GPRS-service and the circuit switched services of the GSM-system. In a physical channel resources can be reused and certain signalling can be common to both. It is possible to reserve in the same carrier wave time slots for circuit switched use and for the packet switched GPRS-use.
FIG. 1 presents telecommunication network connections in a packet switched GPRS-service. The main element of the infrastructure of the network for GPRS-services is a GPRS-support node, so called GSN (GPRS Support Node). It is a mobility router which realizes the connecting and co-operation between different data networks, for instance to PSPDN (Packet Switched Packet Data Network) through interface Gi or to another operator""s GPRS-network through interface Gp, mobility management using GPRS-registers over interface Gr and the transfer of data packets to mobile stations MS independent of their location. It is possible to integrate physically GPRS-node GSN with mobile switching center MSC (Mobile Switching Center) or it can be a separate network element based upon the architecture of data network routers. User data passes directly between support node GSN and base stations system BSS, consisting of base stations BTS and base station controllers BSC, through interface Gb, but between support node GSN and mobile switching center MSC there is signalling interface Gs. In FIG. 1 the uninterrupted lines between blocks represent data traffic (i.e. the transfer of speech or data in a digital form) and the interrupted lines represent signalling. Physically the data can pass transparently over mobile switching center MSC. The radio interface between mobile station MS and the fixed network passes through base station BTS and it has been marked with reference Um. References Abis and A represent the interface with base station BTS and base station controller BSC, and respectively between base station controller BSC and mobile switching center MSC, which is a signalling connection. Reference Gn represents an interface between the different support nodes of the same operator. The support nodes are normally divided into gateway support nodes GGSN (Gateway GSN) and serving or home support nodes SGSN (Serving GSN) as presented FIG. 1.
The GSM-system is a time division multiple access-type (TDMA, Time Division Multiple Access) system, in which the traffic in the radio path is time-divided and takes place in repeated TDMA-frames, each of which consists of several (eight) time slots. In each time slot it is transmitted an information packet in form of a radio frequency burst of finite duration consisting of a number of modulated bits. The time slots are mainly used as control channels and traffic channels. On the traffic channels it is transferred speech and data and in the control channels it is carried out signalling between base station BTS and mobile station MS.
In the following it is explained the protocols of GPRS and the protocol hierarchy in radio interface Um between mobile station MS and a fixed network (home support node SGSN) with reference to FIG. 2a. User data is handled hierarchically on different levels, when it is converted into a form suited for the physical radio path and the public data network. On the highest level A) the user data (coming e.g. from an application App) is in a form suited for the protocol of the public data network, such as TCP/IP and X.25 and on the lowest level E) the data is in a form suited for transferring in the GSM-radio path.
The highest level A) protocol SNDCP (Subnetwork Dependent Convergent Protocol), i.e. a convergence protocol dependent of a subnetwork is explained in more detail in GSM radio specifications 04.65 and 03.60. According to SNDCP a network protocol data unit is segmented between mobile station MS and home support node SGSN into one or several SNDCP data units, the maximum size of the payload of which is approximately 1600 octets. The SNDCP-data unit is transferred in one LLC-fame (Logical Link Control) over the radio interface. The SNDCP-protocol includes multiplexing of user data, segmenting and compressing, and the compressing of the TCP/IP-header. It is possible to transfer in the SNDCP-protocol different network level protocols, such as IP, X.25, PTM-M and PTM-G. The size of a SNDCP user data field is, as to the total number of bits, divisible by eight bits, i.e. it is octet oriented.
The protocol of the next B) level, the LLC-protocol or the logical link control protocol has been explained in more detail in GSM standard specifications 04.64 and 03.60. The LLC-protocol provides a reliable logical link between a mobile station and home support node SGSN. SNDCP-, short messages and GPRS signalling messages are transmitted in LLC-frames which have a frame header containing numbering and a temporary address field, an information field of variable length and a frame check sequence. The functionality of LLC includes maintaining the communication context of mobile station MS and home support node SGSN, the transmitting of acknowledged and unacknowledged frames, the detection and retransmitting of corrupted frames. LLC-frames are transmitted in one or several radio blocks. The logical link is maintained when mobile station MS moves between cells within the area of one home support node SGSN. If mobile station MS moves into the area of another home support node SGSN, a new logical link must be established. The size of a LLC-protocol user data field is, as to the total number of bits, divisible by eight bits, i.e. it is octet oriented.
The next level C) after LLC, the RLC-level (Radio Link Control) has been explained in more detail in GSM standard specifications 03.64. The LLC-frame is being transmitted continuously. The variable length LLC-frames is transmitted in one or more RLC-blocks. The functionality of RLC between mobile station MS and home support node SGSN is to detect the corrupted RLC-blocks and to ask for a selective retransmission of the corrupted blocks. A retransmission request comprises a bit map indicating each air path block to be either corrupted or successfully received. Based upon the bit map the transmitter retransmits the corrupted blocks. The total size of an RLC-block is, the header and user data included, as to the number of bits, is divisible by eight bits, i.e. it is octet oriented.
Also level D), the MAC-level (Medium Access Control) has been explained in more detail in GSM standard specifications 03.64. MAC is used for dividing radio channels between mobile stations and for the allocating of a radio channel for a mobile station for transmission and reception as needed. The functionality of MAC includes a separate header containing uplink state flag USF (Uplink State Flag), block type indicator T and eventual power control information PC (Power Control). The MAC-header and the RLC-data block are placed in radio block RB (see FIGS. 2b and 2c) to be transmitted on the physical layer.
Protocol level E) describes the physical layer or GSM-radio path, in which messages are transferred in radio blocks RB presented in FIGS. 2b and 2c. Radio block RB includes a MAC-header, an information part containing the data or the signalling (RLC-data block, FIG. 2b or an RLC/MAC-signalling-information block, FIG. 2c) and block check sequence BCS (Block Check Sequence). Each radio block is interleaved in four standard bursts. Before the interleaving it is performed a channel coding on the radio block. For the channel coding there are four different coding schemes CS-1, CS-2, CS-3 and CS-4 (Coding Scheme). A mobile station must support all four alternatives. In the channel coding a convolutional coding is performed on the information part. A pre-coding is performed on uplink state flag USF (Uplink State Flag), in which case the length of USF after the pre-coding is dependent on the channel coding method CS-1 . . . CS-4 used. After the channel coding the size of the radio block is according to the GSM-specification 456 bits. Prior to the convolutional coding the payload according to the different coding method varies, and an octet oriented data stream is not achieved with all coding methods CS-1 . . . CS-4. Only CS-1 produces an octet oriented data stream, but the other the channel coding methods CS-2 . . . CS-4 do not do it according to the present protocols. This hampers data stream between different layers A)-E) in mobile station MS and in the mobile communications network, i.e. in base station system BSS and in home support node SGSN.
Now it is introduced such a method, with which data flow can be made simpler between all hierarchy levels or between mobile station MS and the different protocols of mobile communications network BSS; SGSN. This is achieved by bringing the user data flow into octet form on all protocol levels of the GPRS-service, in particular on the lower levels, by setting a certain number of bits as fill bits instead of using them for the transfer of user data. With this method it is possible to make the payload of a radio block octet oriented when any of channel coding methods CS-1 . . . CS-4 is used. A certain number of the bits of radio block RB, the being determined according to the method, are set prior to channel coding and the interleaving of the radio block (in four bursts) to transfer fill bits in such a way that the number of bits in the radio block transferring user data is divisible by eight prior to the channel coding. By using the method the handling of data, in particular that of the user data to be transferred, is made octet oriented on all GPRS-protocol levels. Because the radio block is made octet oriented the operation can after the channel coding be carried out fully in accordance with the GSM-specifications.
If this method were not used, the transmissions of two radio blocks would be mixed in such a way that the last bits of the preceding radio block would be transferred in the same burst with the first bits of the next radio block. This would make the handling of the data and protocols, and the equipment executing them difficult when octets coming from a higher protocol level should be distributed to different blocks on lower protocol levels.
The method according to the invention is characterized in that in the transfer bits of a radio block coded using at least a certain coding method it is transferred user data in a first part of the transfer bits and fill bits in a second part in such a way that it is chosen such a number bits for the of transfer user data which is divisible by eight.
The transmitter/receiver device according to the invention is correspondingly characterized in that it comprises control means for transferring user data in a first part of radio block transfer bits coded using at least a certain coding method and for transferring fill bits in a second part of said transfer bits, and said first part of transfer bits comprises a number of bits divisible by eight.